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The Moon is Earth's only permanent natural satellite.[1]  It is the fifth-largest natural satellite in the Solar System, and the largest among planetary satellites relative to the size of the planet that it orbits (its primary). It is the second-densest satellite among those whose densities are known (after Jupiter's satellite Io).

The average distance of the Moon from the Earth is 384,400 km (238,900 mi), or 1.28 light-seconds.

The Moon is thought to have formed about 4.5 billion years ago, not long after Earth. There are several hypotheses for its origin; the most widely accepted explanation is that the Moon formed from the debris left over after a giant impact between Earth and a Mars-sized body called Theia.

The Moon is in synchronous rotation with Earth, always showing the same face, with its near side marked by dark volcanic maria that fill the spaces between the bright ancient crustal highlands and the prominent impact craters. It is the second-brightest regularly visible celestial object in Earth's sky, after the Sun, as measured by illuminance on Earth's surface. Its surface is actually dark, although compared to the night sky it appears quite bright, with a reflectance just slightly higher than that of worn asphalt. Its prominence in the sky and its regular cycle of phases have made the Moon an important cultural influence after ancient times on language, calendars, art, mythology, and apparently, the menstrual cycles of the female of the human species.

The Moon's gravitational influence produces the ocean tides, body tides, and the slight lengthening of the day. The Moon's current orbital distance is about thirty times the diameter of Earth, with its apparent size in the sky almost the same as that of the Sun, resulting in the Moon covering the Sun nearly precisely in total solar eclipse. This matching of obvious visual size won't continue in the far future. The Moon's linear distance from Earth is currently increasing at a rate of 3.82 ± 0.07 centimetres (1.504 ± 0.028 in) per year, but this rate isn't constant.

The Soviet Union's Luna programme was the first to reach the Moon with uncrewed spacecraft in 1959; the United States' NASA Apollo program achieved the only crewed missions to date, beginning with the first crewed lunar orbiting mission by Apollo 8 in 1968, and six crewed lunar landings between 1969 and 1972, with the first being Apollo 11. These missions returned over 380 kg (840 lb) of lunar rocks, which have been used to develop a geological understanding of the Moon's origin, the formation of its internal structure, and its subsequent history. Since the Apollo 17 mission in 1972, the Moon has been visited only by uncrewed spacecraft.

  

Name and etymology

 

The usual English proper name for Earth's natural satellite is "the Moon". The noun moon in latter day terms is derived from moone (around 1380), which developed from mone (1135), which is derived from Old English mōna (dating from before 725), which ultimately stems from Proto-Germanic *mǣnōn, like all Germanic language cognates. Occasionally, the name "Luna" is used. In literature, especially science fiction, "Luna" is used to distinguish it from additional moons, while in poetry, the name has been used to denote personification of our moon.  In earlier etymology, the words moon and month are both derived from the latin word mensis (month), which derives from an earlier Greek Aeolic form meis/mens (μείς/μηνς) of men/mene (μήν/μήνη; month, moon).[4]   

An abundance of literature including romance involves the moon, reference Romeo's famous soliloquy in Shakespeare's Romeo & Juliet written in 1597.[6]  One of the oldest discovered pictorial references to the moon is a 5,000 years old (3050 B.C. to 2650 B.C.) massive stone monument structure in the shape of a lunar-crescent stone, identified 8 miles northwest of the Sea of Galilee in Israel.[7]   The Bible's Old Testament also makes reference to the Moon.[8]   

The ancient Greeks approximated month with the lunar cycle (lunar cycle is 29-30 days, ref. lunar month), taken using a fundamental 4-year Olympiad basis: The Menai (Menae) [for which ancient untrained travelers ears (for example, traveling to Europe, Germany) might have mistakenly 'heard' moonay or mooney and subsequently shortened to 'moon' via mōna] were fifty goddesses of the lunar months, daughters of Selene (the Moon) and Endymion king of Elis and Olympia, home of the Olympic Games.  The Menai represented the fifty months of the four-year Olympiad cycle--a basic unit in the ancient Greek measurement of time (365.25 x 4 = 1461 days in a modern 4 year cycle versus 1461 divided by 50 "menai" equates to 29.22 days per Menai or moon-lunar cycle 'month'). The eight year Octaeteris--which was used in place of our modern counting of decades--consisted of two Olympiads consisting of fifty and forty-nine lunar months. This 99 lunar month cycle equates to 8 solar years and marks the convergence of the two  heavenly cycles.[5]   Humanity has been gazing at and referencing by the moon for millenniaaeons.   

The principal modern English adjective pertaining to the Moon is lunar, derived from the Latin Luna. A less common adjective is selenic, derived from the Ancient Greek Selene (Σελήνη), from which is derived the prefix "seleno-" (as in selenography). Both the Greek Selene and the Roman goddess Diana were alternatively called Cynthia. The names Luna, Cynthia, and Selene are reflected in terminology for lunar orbits in words such as apolune, pericynthion, and selenocentric. The name Diana is connected to dies meaning 'day'.  See also carpe diem (seize the day) where diem is Latin for day.    

 

Formation

Several mechanisms have been proposed for the Moon's formation 4.53 billion years ago, and a few 30–50 million years after the origin of the Solar System. Recent research presented by Rick Carlson indicates a slightly lower age of between 4.40 and 4.45 billion years. These mechanisms included the fission of the Moon from Earth's crust through centrifugal force (which would require too great an initial spin of Earth), the gravitational capture of a pre-formed Moon (which would require an unfeasibly extended atmosphere of Earth to dissipate the energy of the passing Moon), and the co-formation of Earth and the Moon together in the primordial accretion disk (which doesn't explain the depletion of metals in the Moon). These hypotheses additionally can't account for the high angular momentum of the Earth–Moon system.

File:Evolution of the Moon.ogv
 
The evolution of the Moon and a tour of the Moon

The prevailing hypothesis is that the Earth–Moon system formed as a result of the impact of a Mars-sized body (named Theia) with the proto-Earth Earth (giant impact), that blasted material into orbit about the Earth that then accreted to form the present Earth-Moon system.

This hypothesis, although not perfect, perhaps best explains the evidence. Eighteen months prior to an October 1984 conference on lunar origins, Bill Hartmann, Roger Phillips, and Jeff Taylor challenged fellow lunar scientists: "You have eighteen months. Go back to your Apollo data, go back to your computer, do whatever you have to, but make up your mind. Don't come to our conference unless you have something to say about the Moon's birth." At the 1984 conference at Kona, Hawaii, the giant impact hypothesis emerged as the most popular.

Before the conference, there were partisans of the three "traditional" theories, plus a few people who were starting to take the giant impact seriously, and there was a huge apathetic middle who didn’t think the debate would ever be resolved. Afterward there were essentially only two groups: the giant impact camp and the agnostics.

Giant impacts are thought to have been common in the early Solar System. Computer simulations of a giant impact have produced results that are consistent with the mass of the lunar core and the present angular momentum of the Earth–Moon system. These simulations additionally show that most of the Moon derived from the impactor, rather than the proto-Earth. More recent simulations suggest a larger fraction of the Moon derived from the original Earth mass. Studies of meteorites originating from inner Solar System bodies such as Mars and Vesta show that they have quite different oxygen and tungsten isotopic compositions as compared to Earth, whereas Earth and the Moon have nearly identical isotopic compositions. The isotopic equalisation of the Earth-Moon system might be explained by the post-impact mixing of the vaporised material that formed the two, although this is debated.

The great amount of energy released in the impact event and the subsequent re-accretion of that material into the Earth-Moon system would have melted the outer shell of Earth, forming a magma ocean. Similarly, the newly formed Moon would additionally have been affected and had its own lunar magma ocean; estimates for its depth range from about 500 km (300 miles) to its entire depth (1,737 km (1,079 miles)).

While the giant impact hypothesis might explain a large number of lines of evidence, there are still a few unresolved questions, most of which involve the Moon's composition.

Oceanus Procellarum ("Ocean of Storms")
 
Ancient rift valleys – rectangular structure (visible – topography – GRAIL gravity gradients)
 
Ancient rift valleys – context.
 
Ancient rift valleys – closeup (artist's concept).
 

In 2001, a team at the Carnegie Institute of Washington reported the most precise measurement of the isotopic signatures of lunar rocks. To their surprise, the team found that the rocks from the Apollo program carried an isotopic signature that was identical with rocks from Earth, and were different from almost all additional bodies in the Solar System. Because most of the material that went into orbit to form the Moon was thought to come from Theia, this observation was unexpected. In 2007, researchers from the California Institute of Technology announced that there was less than a one percent chance that Theia and Earth had identical isotopic signatures. Published in 2012, an analysis of titanium isotopes in Apollo lunar samples showed that the Moon has the same composition as Earth, which conflicts with what's expected if the Moon formed far from Earth's orbit or from Theia. Variations on the giant impact hypothesis might explain this data.

Physical characteristics

Internal structure

 
Structure of the Moon
Chemical composition of the lunar surface regolith (derived from crustal rocks)
CompoundFormulaComposition (wt %)
MariaHighlands
silicaSiO245.4%45.5%
aluminaAl2O314.9%24.0%
limeCaO11.8%15.9%
iron(II) oxideFeO14.1%5.9%
magnesiaMgO9.2%7.5%
titanium dioxideTiO23.9%0.6%
sodium oxideNa2O0.6%0.6%
Total99.9%100.0%

The Moon is a differentiated body: it has a geochemically distinct crust, mantle, and core. The Moon has a solid iron-rich inner core with a radius of 240 km (150 mi) and a fluid outer core primarily made of liquid iron with a radius of roughly 300 km (190 mi). Around the core is a partially molten boundary layer with a radius of about 500 km (310 mi). This structure is thought to have developed through the fractional crystallization of a global magma ocean shortly after the Moon's formation 4.5 billion years ago. Crystallization of this magma ocean would have created a mafic mantle from the precipitation and sinking of the minerals olivine, clinopyroxene, and orthopyroxene; after about three-quarters of the magma ocean had crystallised, lower-density plagioclase minerals could form and float into a crust atop. The final liquids to crystallise would have been initially sandwiched between the crust and mantle, with a high abundance of incompatible and heat-producing elements. Consistent with this perspective, geochemical mapping made from orbit suggests the crust of mostly anorthosite. The Moon rock samples of the flood lavas that erupted onto the surface from partial melting in the mantle confirm the mafic mantle composition, which is more iron rich than that of Earth. The crust is on average about 50 km (31 mi) thick.

The Moon is the second-densest satellite in the Solar System, after Io. Notwithstanding the inner core of the Moon is small, with a radius of about 350 km (220 mi) or less, around twenty percent of the radius of the Moon. Its composition isn't well defined, but is probably metallic iron alloyed with a small amount of sulfur and nickel; analyses of the Moon's time-variable rotation suggest that it is at least partly molten.

Surface geology

 
Topography of the Moon

The topography of the Moon has been measured with laser altimetry and stereo image analysis. Its most visible topographic feature is the giant far-side South Pole–Aitken basin, a few 2,240 km (1,390 mi) in diameter, the largest crater on the Moon and the second-largest confirmed impact crater in the Solar System. At 13 km (8.1 mi) deep, its floor is the lowest point on the surface of the Moon. The highest elevations of the Moon's surface are located directly to the northeast, and it has been suggested might have been thickened by the oblique formation impact of the South Pole–Aitken basin. Other large impact basins, such as Imbrium, Serenitatis, Crisium, Smythii, and Orientale, additionally possess regionally low elevations and elevated rims. The far side of the lunar surface is on average about 1.9 km (1.2 mi) higher than that of the near side.

The discovery of fault scarp cliffs by the Lunar Reconnaissance Orbiter suggest that the Moon has shrunk within the past billion years, by about 90 metres (300 ft). Similar shrinkage features exist on Mercury.

Volcanic features

The dark and relatively featureless lunar plains, clearly be seen with the naked eye, are called maria (Latin for "seas"; singular mare), as they were once believed to be filled with water; they're now known to be vast solidified pools of ancient basaltic lava. Although similar to terrestrial basalts, lunar basalts have more iron and no minerals altered by water. The majority of these lavas erupted or flowed into the depressions associated with impact basins. Several geologic provinces containing shield volcanoes and volcanic domes are found within the near side "maria".

 
Evidence of young lunar volcanism

Almost all maria are on the near side of the Moon, and cover thirty-one percent of the surface of the near side, compared with two percent of the far side. This is thought to be due to a concentration of heat-producing elements under the crust on the near side, seen on geochemical maps obtained by Lunar Prospector's gamma-ray spectrometer, which would have caused the underlying mantle to heat up, partially melt, rise to the surface and erupt. Most of the Moon's mare basalts erupted throughout the Imbrian period, 3.0–3.5 billion years ago, although a few radiometrically dated samples are as old as 4.2 billion years. Until recently, the youngest eruptions, dated by crater counting, appeared to have been only 1.2 billion years ago. In 2006, a study of Ina, a tiny depression in Lacus Felicitatis, found jagged, relatively dust-free features that, due to the lack of erosion by infalling debris, appeared to be only 2 million years old. Moonquakes and releases of gas additionally indicate a few continued lunar activity. In 2014 NASA announced "widespread evidence of young lunar volcanism" at 70 irregular mare patches identified by the Lunar Reconnaissance Orbiter, a few less than 50 million years old. This raises the possibility of a much warmer lunar mantle than previously believed, at least on the near side where the deep crust is substantially warmer due to the greater concentration of radioactive elements. Just prior to this, evidence has been presented for 2–10 million years younger basaltic volcanism inside Lowell crater, Orientale basin, located in the transition zone between the near and far sides of the Moon. An initially hotter mantle and/or local enrichment of heat-producing elements in the mantle can be responsible for prolonged activities additionally on the far side in the Orientale basin.

The lighter-coloured regions of the Moon are called terrae, or more commonly highlands, because they're higher than most maria. They have been radiometrically dated to having formed 4.4 billion years ago, and might represent plagioclase cumulates of the lunar magma ocean. In contrast to Earth, no major lunar mountains are believed to have formed as a result of tectonic events.

The concentration of maria on the Near Side likely reflects the substantially thicker crust of the highlands of the Far Side, which might have formed in a slow-velocity impact of a second moon of Earth a few tens of millions of years after their formation.

Impact craters

 
Lunar crater Daedalus on the Moon's far side

The additional major geologic process that has affected the Moon's surface is impact cratering, with craters formed when asteroids and comets collide with the lunar surface. There are estimated to be roughly 300,000 craters wider than 1 km (0.6 mi) on the Moon's near side alone. The lunar geologic timescale is based on the most prominent impact events, including Nectaris, Imbrium, and Orientale, structures characterised by multiple rings of uplifted material, between hundreds and thousands of km in diameter and associated with a broad apron of ejecta deposits that form a regional stratigraphic horizon. The lack of an atmosphere, weather and recent geological processes mean that a large number of of these craters are well-preserved. Although only a few multi-ring basins have been definitively dated, they're useful for assigning relative ages. Because impact craters accumulate at a nearly constant rate, counting the number of craters per unit area can be used to estimate the age of the surface. The radiometric ages of impact-melted rocks collected throughout the Apollo missions cluster between 3.8 and 4.1 billion years old: this has been used to propose a Late Heavy Bombardment of impacts.

Blanketed on top of the Moon's crust is a highly comminuted (broken into ever smaller particles) and impact gardened surface layer called regolith, formed by impact processes. The finer regolith, the lunar soil of silicon dioxide glass, has a texture resembling snow and a scent resembling spent gunpowder. The regolith of older surfaces is generally thicker than for younger surfaces: it varies in thickness from 10–20 km (6.2–12.4 mi) in the highlands and 3–5 km (1.9–3.1 mi) in the maria. Beneath the finely comminuted regolith layer is the megaregolith, a layer of highly fractured bedrock a large number of km thick.

Comparison of high-resolution images obtained by the Lunar Reconnaissance Orbiter has shown a contemporary crater-production rate significantly higher than previously estimated. A secondary cratering process caused by distal ejecta is thought to churn the top two centimetres of regolith a hundred times more quickly than previous models suggested–on a timescale of 81,000 years.

 
Lunar swirls at Reiner Gamma

Lunar swirls

Lunar swirls are enigmatic features found across the Moon's surface, which are characterised by a high albedo, appearing optically immature (i.e. the optical characteristics of a relatively young regolith), and often displaying a sinuous shape. Their curvilinear shape is often accentuated by low albedo regions that wind between the bright swirls.

Presence of water

Liquid water can't persist on the lunar surface. When exposed to solar radiation, water quickly decomposes through a process known as photodissociation and is lost to space. Notwithstanding after the 1960s, scientists have hypothesised that water ice might be deposited by impacting comets or possibly produced by the reaction of oxygen-rich lunar rocks, and hydrogen from solar wind, leaving traces of water which could possibly survive in cold, permanently shadowed craters at either pole on the Moon. Computer simulations suggest that up to 14,000 km2 (5,400 sq mi) of the surface might be in permanent shadow. The presence of usable quantities of water on the Moon is an important factor in rendering lunar habitation as a cost-effective plan; the alternative of transporting water from Earth would be prohibitively expensive.

In years since, signatures of water have been found to exist on the lunar surface. In 1994, the bistatic radar experiment located on the Clementine spacecraft, indicated the existence of small, frozen pockets of water close to the surface. Notwithstanding later radar observations by Arecibo, suggest these findings might rather be rocks ejected from young impact craters. In 1998, the neutron spectrometer on the Lunar Prospector spacecraft, showed that high concentrations of hydrogen are present in the first metre of depth in the regolith near the polar regions. Volcanic lava beads, brought back to Earth aboard Apollo 15, showed small amounts of water in their interior.

The 2008 Chandrayaan-1 spacecraft has after confirmed the existence of surface water ice, using the on-board Moon Mineralogy Mapper. The spectrometer observed absorption lines common to hydroxyl, in reflected sunlight, providing evidence of large quantities of water ice, on the lunar surface. The spacecraft showed that concentrations might possibly be as high as 1,000 ppm. In 2009, LCROSS sent a 2,300 kg (5,100 lb) impactor into a permanently shadowed polar crater, and detected at least 100 kg (220 lb) of water in a plume of ejected material. An Additional examination of the LCROSS data showed the amount of detected water to be closer to 155 ± 12 kg (342 ± 26 lb).

In May 2011, 615–1410 ppm water in melt inclusions in lunar sample 74220 was reported, the famous high-titanium "orange glass soil" of volcanic origin collected throughout the Apollo 17 mission in 1972. The inclusions were formed throughout explosive eruptions on the Moon approximately 3.7 billion years ago. This concentration is comparable with that of magma in Earth's upper mantle. Although of considerable selenological interest, Hauri's announcement affords little comfort to would-be lunar colonists—the sample originated a large number of km below the surface, and the inclusions are so difficult to access that it took 39 years to find them with a state-of-the-art ion microprobe instrument.

Gravitational field

 
GRAIL's gravity map of the Moon

The gravitational field of the Moon has been measured through tracking the Doppler shift of radio signals emitted by orbiting spacecraft. The main lunar gravity features are mascons, large positive gravitational anomalies associated with a few of the giant impact basins, partly caused by the dense mare basaltic lava flows that fill those basins. The anomalies greatly influence the orbit of spacecraft about the Moon. There are a few puzzles: lava flows by themselves can't explain all of the gravitational signature, and a few mascons exist that aren't linked to mare volcanism.

Magnetic field

The Moon has an external magnetic field of about 1–100 nanoteslas, less than one-hundredth that of Earth. It doesn't currently have a global dipolar magnetic field and only has crustal magnetization, probably acquired early in lunar history when a dynamo was still operating. Alternatively, a few of the remnant magnetization might be from transient magnetic fields generated throughout large impact events through the expansion of an impact-generated plasma cloud in the presence of an ambient magnetic field. This is supported by the obvious location of the largest crustal magnetizations near the antipodes of the giant impact basins.

Atmosphere

 
Sketch by the Apollo 17 astronauts. The lunar atmosphere was later studied by LADEE.

The Moon has an atmosphere so tenuous as to be nearly vacuum, with a total mass of less than 10 metric tonnes (9.8 long tons; 11 short tons). The surface pressure of this small mass is around 3 × 10−15 atm (0.3 nPa); it varies with the lunar day. Its sources include outgassing and sputtering, a product of the bombardment of lunar soil by solar wind ions. Elements that have been detected include sodium and potassium, produced by sputtering (also found in the atmospheres of Mercury and Io); helium-4 and neon from the solar wind; and argon-40, radon-222, and polonium-210, outgassed after their creation by radioactive decay within the crust and mantle. The absence of such neutral species (atoms or molecules) as oxygen, nitrogen, carbon, hydrogen and magnesium, which are present in the regolith, isn't understood. Water vapour has been detected by Chandrayaan-1 and found to vary with latitude, with a maximum at ~60–70 degrees; it is possibly generated from the sublimation of water ice in the regolith. These gases either return into the regolith due to the Moon's gravity or be lost to space, either through solar radiation pressure or, if they're ionized, by being swept away by the solar wind's magnetic field.

Dust

A permanent asymmetric moon dust cloud exists around the Moon, created by small particles from comets. Estimates are 5 tonnes of comet particles strike the Moon's surface each 24 hours. The particles strike the Moon's surface ejecting moon dust above the Moon. The dust stays above the Moon approximately 10 minutes, taking 5 minutes to rise, and 5 minutes to fall. On average, 120 kilogrammes of dust are present above the Moon, rising to 100 km above the surface. The dust measurements were made by LADEE's Lunar Dust EXperiment (LDEX), between 20 and 100 km above the surface, throughout a six-month period. LDEX detected an average of one 0.3 micrometre moon dust particle each minute. Dust particle counts peaked throughout the Geminid, Quadrantid, Northern Taurid, and Omicron Centaurid meteor showers, when the Earth, and Moon, pass through comet debris. The cloud is asymmetric, more dense near the boundary between the Moon's dayside and nightside.

Seasons

The Moon's axial tilt with respect to the ecliptic is only 1.5424°, much less than the 23.44° of Earth. Because of this, the Moon's solar illumination varies much less with season, and topographical details play a crucial role in seasonal effects. From images taken by Clementine in 1994, it appears that four mountainous regions on the rim of Peary Crater at the Moon's north pole might remain illuminated for the entire lunar day, creating peaks of eternal light. No such regions exist at the south pole. Similarly, there are places that remain in permanent shadow at the bottoms of a large number of polar craters, and these dark craters are extremely cold: Lunar Reconnaissance Orbiter measured the lowest summer temperatures in craters at the southern pole at 35 K (−238 °C; −397 °F) and just 26 K (−247 °C; −413 °F) close to the winter solstice in north polar Hermite Crater. This is the coldest temperature in the Solar System ever measured by a spacecraft, colder even than the surface of Pluto. Average temperatures of the Moon's surface are reported, but temperatures of different areas will vary greatly depending upon whether it is in sunlight or shadow.

Relationship to Earth

Orbit

 
Earth–Moon system (schematic)
 
DSCOVR satellite sees the Moon passing in front of Earth

The Moon makes a complete orbit around Earth with respect to the fixed stars about once every 27.3 days (its sidereal period). Notwithstanding because Earth is moving in its orbit around the Sun at the same time, it takes slightly longer for the Moon to show the same phase to Earth, which is about 29.5 days (its synodic period). Unlike most satellites of additional planets, the Moon orbits closer to the ecliptic plane than to the planet's equatorial plane. The Moon's orbit is subtly perturbed by the Sun and Earth in a large number of small, complex and interacting ways. For example, the plane of the Moon's orbital motion gradually rotates, which affects additional aspects of lunar motion. These follow-on effects are mathematically described by Cassini's laws.

Relative size

The Moon is exceptionally large relative to Earth: a quarter its diameter and 1/81 its mass. It is the largest moon in the Solar System relative to the size of its planet, though Charon is larger relative to the dwarf planet Pluto, at 1/9 Pluto's mass. Earth and the Moon are nevertheless still considered a planet–satellite system, rather than a double planet, because their barycentre, the common centre of mass, is located 1,700 km (1,100 mi) (about a quarter of Earth's radius) beneath Earth's surface.

Appearance from Earth

 
Moon setting in western sky over the High Desert in California

The Moon is in synchronous rotation: it rotates about its axis in about the same time it takes to orbit Earth. This results in it nearly always keeping the same face turned towards Earth. The Moon used to rotate at a faster rate, but early in its history, its rotation slowed and became tidally locked in this orientation as a result of frictional effects associated with tidal deformations caused by Earth. With time, the energy of rotation of the Moon on its axis was dissipated as heat, until there was no rotation of the Moon relative to the Earth. The side of the Moon that faces Earth is called the near side, and the opposite the far side. The far side is often inaccurately called the "dark side", but it is in fact illuminated as often as the near side: once per lunar day, throughout the new moon phase we observe on Earth when the near side is dark. In 2016, planetary scientists, using data collected on the much earlier Nasa Lunar Prospector mission, found two hydrogen-rich areas on opposite sides of the Moon, probably in the form of water ice. It is speculated that these patches were the poles of the Moon billions of years ago, before it was tidally locked to Earth.

The Moon has an exceptionally low albedo, giving it a reflectance that's slightly brighter than that of worn asphalt. Despite this, it is the brightest object in the sky after the Sun. This is partly due to the brightness enhancement of the opposition effect; at quarter phase, the Moon is only one-tenth as bright, rather than half as bright, as at full moon.

Additionally, colour constancy in the visual system recalibrates the relations between the colours of an object and its surroundings, and because the surrounding sky is comparatively dark, the sunlit Moon is perceived as a bright object. The edges of the full moon seem as bright as the centre, with no limb darkening, due to the reflective properties of lunar soil, which reflects more light back towards the Sun than in additional directions. The Moon does appear larger when close to the horizon, but this is a purely psychological effect, known as the Moon illusion, first described in the seventh century BC. The full moon subtends an arc of about 0.52° (on average) in the sky, roughly the same obvious size as the Sun (see ).

The highest altitude of the Moon in the sky varies with the lunar phase and the season of the year. The full moon is highest throughout winter. The 18.6-year nodes cycle additionally has an influence: when the ascending node of the lunar orbit is in the vernal equinox, the lunar declination can go as far as 28° each month. This means the Moon can go overhead at latitudes up to 28° from the equator, instead of only 18°. The orientation of the Moon's crescent additionally depends on the latitude of the observation site: close to the equator, an observer can see a smile-shaped crescent moon.

The Moon is visible for two weeks every 27.3 days at the North and South Pole. The Moon's light is used by zooplankton in the Arctic when the sun is below the horizon for months on end.

The distance between the Moon and Earth varies from around 356,400 km (221,500 mi) to 406,700 km (252,700 mi) at perigees (closest) and apogees (farthest), respectively. On 19 March 2011, it was closer to Earth when at full phase than it has been after 1993, fourteen percent closer than its farthest position in apogee. Reported as a "super moon", this closest point coincides within an hour of a full moon, and it was thirty percent more luminous than when at its greatest distance due to its angular diameter being fourteen percent greater, because scriptstyle 1.14^{2}approx 1.30. At lower levels, the human perception of reduced brightness as a percentage is provided by the following formula:

{displaystyle {text{perceived reduction}}%=100times {sqrt {{text{actual reduction}}% over 100}}}

When the actual reduction is 1.00 / 1.30, or about 0.770, the perceived reduction is about 0.877, or 1.00 / 1.14. This gives a maximum perceived increase of fourteen percent between apogee and perigee moons of the same phase.

There has been historical controversy over whether features on the Moon's surface change over time. Today, a large number of of these claims are thought to be illusory, resulting from observation under different lighting conditions, poor astronomical seeing, or inadequate drawings. Notwithstanding outgassing does occasionally occur, and can be responsible for a minor percentage of the reported lunar transient phenomena. Recently, it has been suggested that a roughly 3 km (1.9 mi) diameter region of the lunar surface was modified by a gas release event about a million years ago. The Moon's appearance, like that of the Sun, can be affected by Earth's atmosphere: common effects are a 22° halo ring formed when the Moon's light is refracted through the ice crystals of high cirrostratus cloud, and smaller coronal rings when the Moon is seen through thin clouds.

 
 
The monthly changes of angle between the direction of illumination by the Sun and viewing from Earth, and the phases of the Moon that result

The illuminated area of the visible sphere (degree of illumination) is given by {frac {1}{2}}(1-cos e), where e is the elongation (i.e. the angle between Moon, the observer (on Earth) and the Sun).

Tidal effects

 
The libration of the Moon over a single lunar month. Also visible is the slight variation in the Moon's visual size from Earth.

The gravitational attraction that masses have for one another decreases inversely with the square of the distance of those masses from each other. As a result, the slightly greater attraction that the Moon has for the side of Earth closest to the Moon, as compared to the part of the Earth opposite the Moon, results in tidal forces. Tidal forces affect both the Earth's crust and oceans.

The most obvious effect of tidal forces is to cause two bulges in the Earth's oceans, one on the side facing the Moon and the additional on the side opposite. This results in elevated sea levels called ocean tides. As the Earth spins on its axis, one of the ocean bulges (high tide) is held in place "under" the Moon, while another such tide is opposite. As a result, there are two high tides, and two low tides in about 24 hours. Since the Moon is orbiting the Earth in the same direction of the Earth's rotation, the high tides occur about every 12 hours and 25 minutes; the 25 minutes is due to the Moon's time to orbit the Earth. The Sun has the same tidal effect on the Earth, but its forces of attraction are only forty percent that of the Moon's; the Sun's and Moon's interplay is responsible for spring and neap tides. If the Earth was a water world (one with no continents) it would produce a tide of only one meter, and that tide would be quite predictable, but the ocean tides are greatly modified by additional effects: the frictional coupling of water to Earth's rotation through the ocean floors, the inertia of water's movement, ocean basins that grow shallower near land, the sloshing of water between different ocean basins. As a result, the timing of the tides at most points on the Earth is a product of observations that are explained, incidentally, by theory.

While gravitation causes acceleration and movement of the Earth's fluid oceans, gravitational coupling between the Moon and Earth's solid body is mostly elastic and plastic. The result is a further tidal effect of the Moon on the Earth that causes a bulge of the solid portion of the Earth nearest the Moon that acts as a torque in opposition to the Earth's rotation. This "drains" angular momentum and rotational kinetic energy from Earth's spin, slowing the Earth's rotation. That angular momentum, lost from the Earth, is transferred to the Moon in a process (confusingly known as tidal acceleration), which lifts the Moon into a higher orbit and results in its lower orbital speed about the Earth. Thus the distance between Earth and Moon is increasing, and the Earth's spin is slowing in reaction. Measurements from laser reflectors left throughout the Apollo missions (lunar ranging experiments) have found that the Moon's distance increases by 38 mm (1.5 in) per year (roughly the rate at which human fingernails grow). Atomic clocks additionally show that Earth's day lengthens by about 15 microseconds every year, slowly increasing the rate at which UTC is adjusted by leap seconds. Left to run its course, this tidal drag would continue until the spin of Earth and the orbital period of the Moon matched, creating mutual tidal locking between the two. As a result, the Moon would be suspended in the sky over one meridian, as is already currently the case with Pluto and its moon Charon. Notwithstanding the Sun will become a red giant long before that, engulfing Earth and we need not worry about the consequences.

In a like manner, the lunar surface experiences tides of around 10 cm (4 in) amplitude over 27 days, with two components: a fixed one due to Earth, because they're in synchronous rotation, and a varying component from the Sun. The Earth-induced component arises from libration, a result of the Moon's orbital eccentricity (if the Moon's orbit were perfectly circular, there would only be solar tides). Libration additionally changes the angle from which the Moon is seen, allowing a total of about 59 percent of its surface to be seen from Earth over time. The cumulative effects of stress built up by these tidal forces produces moonquakes. Moonquakes are much less common and weaker than are earthquakes, although moon quakes can last for up to an hour—a significantly longer time than terrestrial quakes—because of the absence of water to damp out the seismic vibrations. The existence of moonquakes was an unexpected discovery from seismometers placed on the Moon by Apollo astronauts from 1969 through 1972.

Eclipses

 
 
 
From Earth, the Moon and the Sun appear the same size, as seen in the 1999 solar eclipse (left), whereas from the STEREO-B spacecraft in an Earth-trailing orbit, the Moon appears much smaller than the Sun (right).

Eclipses can only occur when the Sun, Earth, and Moon are all in a straight line (termed "syzygy"). Solar eclipses occur at new moon, when the Moon is between the Sun and Earth. In contrast, lunar eclipses occur at full moon, when Earth is between the Sun and Moon. The obvious size of the Moon is roughly the same as that of the Sun, with both being viewed at close to one-half a degree wide. The Sun is much larger than the Moon but it is the precise vastly greater distance that gives it the same obvious size as the much closer and much smaller Moon from the perspective of Earth. The variations in obvious size, due to the non-circular orbits, are nearly the same as well, though occurring in different cycles. This makes possible both total (with the Moon appearing larger than the Sun) and annular (with the Moon appearing smaller than the Sun) solar eclipses. In a total eclipse, the Moon completely covers the disc of the Sun and the solar corona becomes visible to the naked eye. Because the distance between the Moon and Earth is quite slowly increasing over time, the angular diameter of the Moon is decreasing. Also, as it evolves toward fitting a red giant, the size of the Sun, and its obvious diameter in the sky, are slowly increasing. The combination of these two changes means that hundreds of millions of years ago, the Moon would always completely cover the Sun on solar eclipses, and no annular eclipses were possible. Likewise, hundreds of millions of years in the future, the Moon will no longer cover the Sun completely, and total solar eclipses won't occur.

Because the Moon's orbit around Earth is inclined by about 5° to the orbit of Earth around the Sun, eclipses don't occur at every full and new moon. For an eclipse to occur, the Moon must be near the intersection of the two orbital planes. The periodicity and recurrence of eclipses of the Sun by the Moon, and of the Moon by Earth, is described by the saros, which has a period of approximately 18 years.

Because the Moon is continuously blocking our view of a half-degree-wide circular area of the sky, the related phenomenon of occultation occurs when a bright star or planet passes behind the Moon and is occulted: hidden from view. In this way, a solar eclipse is an occultation of the Sun. Because the Moon is comparatively close to Earth, occultations of individual stars aren't visible everywhere on the planet, nor at the same time. Because of the precession of the lunar orbit, each year different stars are occulted.

Observation and exploration

Ancient and mediaeval studies

 
Map of the Moon by Johannes Hevelius from his Selenographia (1647), the first map to include the libration zones
 
A study of the Moon in Robert Hooke's Micrographia, 1665

Understanding of the Moon's cycles was an early development of astronomy: by the 5th century BC, Babylonian astronomers had recorded the 18-year Saros cycle of lunar eclipses, and Indian astronomers had described the Moon's monthly elongation. The Chinese astronomer Shi Shen (fl. fourth century BC) gave instructions for predicting solar and lunar eclipses. Later, the physical form of the Moon and the cause of moonlight became understood. The ancient Greek philosopher Anaxagoras (d. 428 BC) reasoned that the Sun and Moon were both giant spherical rocks, and that the latter reflected the light of the former. Although the Chinese of the Han Dynasty believed the Moon to be energy equated to qi, their 'radiating influence' theory additionally recognised that the light of the Moon was merely a reflection of the Sun, and Jing Fang (78–37 BC) noted the sphericity of the Moon. In the second century AD Lucian wrote a novel where the heroes travel to the Moon, which is inhabited. In 499 AD, the Indian astronomer Aryabhata mentioned in his Aryabhatiya that reflected sunlight is the cause of the shining of the Moon. The astronomer and physicist Alhazen (965–1039) found that sunlight wasn't reflected from the Moon like a mirror, but that light was emitted from every part of the Moon's sunlit surface in all directions. Shen Kuo (1031–1095) of the Song dynasty created an allegory equating the waxing and waning of the Moon to a round ball of reflective silver that, when doused with white powder and viewed from the side, would appear to be a crescent.

 
Galileo's sketches of the Moon from Sidereus Nuncius

In Aristotle's (384–322 BC) description of the universe, the Moon marked the boundary between the spheres of the mutable elements (earth, water, air and fire), and the imperishable stars of aether, an influential philosophy that would dominate for centuries. Notwithstanding in the 2nd century BC, Seleucus of Seleucia correctly theorised that tides were due to the attraction of the Moon, and that their height depends on the Moon's position relative to the Sun. In the same century, Aristarchus computed the size and distance of the Moon from Earth, obtaining a value of about twenty times the radius of Earth for the distance. These figures were greatly improved by Ptolemy (90–168 AD): his values of a mean distance of 59 times Earth's radius and a diameter of 0.292 Earth diameters were close to the correct values of about 60 and 0.273 respectively. Archimedes (287–212 BC) designed a planetarium that could calculate the motions of the Moon and additional objects in the Solar System.

During the Middle Ages, before the invention of the telescope, the Moon was increasingly recognised as a sphere, though a large number of believed that it was "perfectly smooth".

In 1609, Galileo Galilei drew one of the first telescopic drawings of the Moon in his book Sidereus Nuncius and noted that it wasn't smooth but had mountains and craters. Telescopic mapping of the Moon followed: later in the seventeenth century, the efforts of Giovanni Battista Riccioli and Francesco Maria Grimaldi led to the system of naming of lunar features in use today. The more exact 1834–36 Mappa Selenographica of Wilhelm Beer and Johann Heinrich Mädler, and their associated 1837 book Der Mond, the first trigonometrically accurate study of lunar features, included the heights of more than a thousand mountains, and introduced the study of the Moon at accuracies possible in earthly geography. Lunar craters, first noted by Galileo, were thought to be volcanic until the 1870s proposal of Richard Proctor that they were formed by collisions. This view gained support in 1892 from the experimentation of geologist Grove Karl Gilbert, and from comparative studies from 1920 to the 1940s, leading to the development of lunar stratigraphy, which by the 1950s was fitting a new and growing branch of astrogeology.

By spacecraft

20th century

Soviet missions
 
 
 
Luna 2, the first human-made object to reach the surface of the Moon (left) and Soviet Moon rover Lunokhod 1

The Cold War-inspired Space Race between the Soviet Union and the U.S. led to an acceleration of interest in exploration of the Moon. Once launchers had the necessary capabilities, these nations sent uncrewed probes on both flyby and impact/lander missions. Spacecraft from the Soviet Union's Luna program were the first to achieve a number of goals: following three unnamed, failed missions in 1958, the first human-made object to escape Earth's gravity and pass near the Moon was Luna 1; the first human-made object to impact the lunar surface was Luna 2, and the first photographs of the normally occluded far side of the Moon were made by Luna 3, all in 1959.

The first spacecraft to perform a successful lunar soft landing was Luna 9 and the first uncrewed vehicle to orbit the Moon was Luna 10, both in 1966. Rock and soil samples were brought back to Earth by three Luna sample return missions (Luna 16 in 1970, Luna 20 in 1972, and Luna 24 in 1976), which returned 0.3 kg total. Two pioneering robotic rovers landed on the Moon in 1970 and 1973 as a part of Soviet Lunokhod programme.

United States missions

The United States launched uncrewed probes to develop an understanding of the lunar surface for an eventual crewed landing: the Jet Propulsion Laboratory's Ranger program produced the first close-up pictures; the Lunar Orbiter program produced maps of the entire Moon; the Surveyor program landed its first spacecraft four months after Luna 9. NASA's crewed Apollo program was developed in parallel; after a series of uncrewed and crewed tests of the Apollo spacecraft in Earth orbit, and spurred on by a potential Soviet lunar flight, in 1968 Apollo 8 made the first crewed mission to lunar orbit. The subsequent landing of the first humans on the Moon in 1969 is seen by a large number of as the culmination of the Space Race.

 
Neil Armstrong working at the lunar module

Neil Armstrong became the first person to walk on the Moon as the commander of the American mission Apollo 11 by first setting foot on the Moon at 02:56 UTC on 21 July 1969. An estimated 500 million people worldwide watched the transmission by the Apollo TV camera, the largest television audience for a live broadcast at that time. The Apollo missions 11 to 17 (except Apollo 13, which aborted its planned lunar landing) returned 380.05 kilogrammes (837.87 lb) of lunar rock and soil in 2,196 separate samples. The American Moon landing and return was enabled by considerable technological advances in the early 1960s, in domains such as ablation chemistry, software engineering and atmospheric re-entry technology, and by highly competent management of the enormous technical undertaking.

Scientific instrument packages were installed on the lunar surface throughout all the Apollo landings. Long-lived instrument stations, including heat flow probes, seismometers, and magnetometers, were installed at the Apollo 12, 14, 15, 16, and 17 landing sites. Direct transmission of data to Earth concluded in late 1977 due to budgetary considerations, but as the stations' lunar laser ranging corner-cube retroreflector arrays are passive instruments, they're still being used. Ranging to the stations is routinely performed from Earth-based stations with an accuracy of a few centimetres, and data from this experiment are being used to place constraints on the size of the lunar core.

1980s–2000
 
An artificially coloured mosaic constructed from a series of 53 images taken through three spectral filters by Galileo' s imaging system as the spacecraft flew over the northern regions of the Moon on December 7, 1992.

After the first Moon race there were years of near quietude but starting in the 1990s, a large number of more countries have become involved in direct exploration of the Moon. In 1990, Japan became the third country to place a spacecraft into lunar orbit with its Hiten spacecraft. The spacecraft released a smaller probe, Hagoromo, in lunar orbit, but the transmitter failed, preventing further scientific use of the mission. In 1994, the U.S. sent the joint Defense Department/NASA spacecraft Clementine to lunar orbit. This mission obtained the first near-global topographic map of the Moon, and the first global multispectral images of the lunar surface. This was followed in 1998 by the Lunar Prospector mission, whose instruments indicated the presence of excess hydrogen at the lunar poles, which is likely to have been caused by the presence of water ice in the upper few metres of the regolith within permanently shadowed craters.

India, Japan, China, the United States, and the European Space Agency each sent lunar orbiters, especially ISRO's Chandrayaan-1 has contributed to confirming the discovery of lunar water ice in permanently shadowed craters at the poles and bound into the lunar regolith. The post-Apollo era has additionally seen two rover missions: the final Soviet Lunokhod mission in 1973, and China's ongoing Chang'e 3 mission, which deployed its Yutu rover on 14 December 2013. The Moon remains, under the Outer Space Treaty, free to all nations to explore for peaceful purposes.

21st century

The European spacecraft SMART-1, the second ion-propelled spacecraft, was in lunar orbit from 15 November 2004 until its lunar impact on 3 September 2006, and made the first detailed survey of chemical elements on the lunar surface.

China has pursued an ambitious program of lunar exploration, beginning with Chang'e 1, which successfully orbited the Moon from 5 November 2007 until its controlled lunar impact on 1 March 2009. In its sixteen-month mission, it obtained a full image map of the Moon. China followed up this success with Chang'e 2 beginning in October 2010, which reached the Moon over twice as fast as Chang'e 1, mapped the Moon at a higher resolution over an eight-month period, then left lunar orbit in favour of an extended stay at the Earth–Sun L2 Lagrangian point, before finally performing a flyby of asteroid 4179 Toutatis on 13 December 2012, and then heading off into deep space. On 14 December 2013, Chang'e 3 improved upon its orbital mission predecessors by landing a lunar lander onto the Moon's surface, which in turn deployed a lunar rover, named Yutu (Chinese: 玉兔; literally "Jade Rabbit"). In so doing, Chang'e 3 made the first lunar soft landing after Luna 24 in 1976, and the first lunar rover mission after Lunokhod 2 in 1973. China intends to launch another rover mission (Chang'e 4) before 2020, followed by a sample return mission (Chang'e 5) soon after.

Between 4 October 2007 and 10 June 2009, the Japan Aerospace Exploration Agency's Kaguya (Selene) mission, a lunar orbiter fitted with a high-definition video camera, and two small radio-transmitter satellites, obtained lunar geophysics data and took the first high-definition movies from beyond Earth orbit. India's first lunar mission, Chandrayaan I, orbited from 8 November 2008 until loss of contact on 27 August 2009, creating a high resolution chemical, mineralogical and photo-geological map of the lunar surface, and confirming the presence of water molecules in lunar soil. The Indian Space Research Organisation planned to launch Chandrayaan II in 2013, which would have included a Russian robotic lunar rover. Notwithstanding the failure of Russia's Fobos-Grunt mission has delayed this project.

 
Copernicus's central peaks as observed by the LRO, 2012
 
The Ina formation, 2009

The U.S. co-launched the Lunar Reconnaissance Orbiter (LRO) and the LCROSS impactor and follow-up observation orbiter on 18 June 2009; LCROSS completed its mission by making a planned and widely observed impact in the crater Cabeus on 9 October 2009, whereas LRO is currently in operation, obtaining precise lunar altimetry and high-resolution imagery. In November 2011, the LRO passed over the Aristarchus crater, which spans 40 km (25 mi) and sinks more than 3.5 km (2.2 mi) deep. The crater is one of the most visible ones from Earth. "The Aristarchus plateau is one of the most geologically diverse places on the Moon: a mysterious raised flat plateau, a giant rille carved by enormous outpourings of lava, fields of explosive volcanic ash, and all surrounded by massive flood basalts", said Mark Robinson, principal investigator of the Lunar Reconnaissance Orbiter Camera at Arizona State University. NASA released photos of the crater on 25 December 2011.

Two NASA GRAIL spacecraft began orbiting the Moon around 1 January 2012, on a mission to learn more about the Moon's internal structure. NASA's LADEE probe, designed to study the lunar exosphere, achieved orbit on 6 October 2013.

Upcoming lunar missions include Russia's Luna-Glob: an uncrewed lander with a set of seismometers, and an orbiter based on its failed Martian Fobos-Grunt mission. Privately funded lunar exploration has been promoted by the Google Lunar X Prize, announced 13 September 2007, which offers US$20 million to anyone who can land a robotic rover on the Moon and meet additional specified criteria. Shackleton Energy Company is building a programme to establish operations on the south pole of the Moon to harvest water and supply their Propellant Depots.

NASA began to plan to resume crewed missions following the call by U.S. President George W. Bush on 14 January 2004 for a crewed mission to the Moon by 2019 and the construction of a lunar base by 2024. The Constellation program was funded and construction and testing begun on a crewed spacecraft and launch vehicle, and design studies for a lunar base. Notwithstanding that programme has been cancelled in favour of a crewed asteroid landing by 2025 and a crewed Mars orbit by 2035. India has additionally expressed its hope to send a crewed mission to the Moon by 2020.

Astronomy from the Moon

For a large number of years, the Moon has been recognised as an excellent site for telescopes. It is relatively nearby; astronomical seeing isn't a concern; certain craters near the poles are permanently dark and cold, and thus especially useful for infrared telescopes; and radio telescopes on the far side would be shielded from the radio chatter of Earth. The lunar soil, although it poses a problem for any moving parts of telescopes, can be mixed with carbon nanotubes and epoxies and employed in the construction of mirrors up to 50 metres in diameter. A lunar zenith telescope can be made cheaply with ionic liquid.

In April 1972, the Apollo 16 mission recorded various astronomical photos and spectra in ultraviolet with the Far Ultraviolet Camera/Spectrograph.

Legal status

During the Cold War, the United States Army conducted a classified feasibility study in the late 1950s called Project Horizon, to construct a crewed military outpost on the Moon, which would have been home to a bombing system targeted at rivals on Earth. The study included the possibility of conducting a lunar-based nuclear test. The Air Force, which at the time was in competition with the Army for a leading role in the space program, developed its own, similar plan called Lunex. Notwithstanding both these proposals were ultimately passed over as the space programme was largely transferred from the military to the civilian agency NASA.

Although Luna landers scattered pennants of the Soviet Union on the Moon, and U.S. flags were symbolically planted at their landing sites by the Apollo astronauts, no nation claims ownership of any part of the Moon's surface. Russia and the U.S. are party to the 1967 Outer Space Treaty, which defines the Moon and all outer space as the "province of all mankind". This treaty additionally restricts the use of the Moon to peaceful purposes, explicitly banning military installations and weapons of mass destruction. The 1979 Moon Agreement was created to restrict the exploitation of the Moon's resources by any single nation, but as of 2014, it has been signed and ratified by only 16 nations, none of which engages in self-launched human space exploration or has plans to do so. Although several individuals have made claims to the Moon in whole or in part, none of these are considered credible.

In culture

 
Luna, the Moon, from a 1550 edition of Guido Bonatti's Liber astronomiae

Mythology

 
Sun and Moon with faces (1493 woodcut)

The Moon was often personified as a lunar deity in mythology and religion. A 5,000-year-old rock carving at Knowth, Ireland, might represent the Moon, which would be the earliest depiction discovered. The contrast between the brighter highlands and the darker maria creates the patterns seen by different cultures as the Man in the Moon, the rabbit and the buffalo, among others. In a large number of prehistoric and ancient cultures, the Moon was personified as a deity or additional supernatural phenomenon, and astrological views of the Moon continue to be propagated today.

In the Ancient Near East, the moon god (Sin/Nanna) was masculine. In Greco-Roman mythology, Sun and Moon are represented as male and female, respectively (Helios/Sol and Selene/Luna). The crescent shape form an early time was used as a symbol representing the Moon. The Moon goddess Selene was represented as wearing a crescent on her headgear in an arrangement reminiscent of horns. The star and crescent arrangement additionally goes back to the Bronze Age, representing either the Sun and Moon, or the Moon and planet Venus, in combination. It came to represent the goddess Artemis or Hecate, and via the patronage of Hecate came to be used as a symbol of Byzantium.

An iconographic tradition of representing Sun and Moon with faces developed in the late mediaeval period.

The splitting of the moon (Arabic: انشقاق القمر‎‎) is a miracle attributed to Muhammad.

Calendar

The Moon's regular phases make it a quite convenient timepiece, and the periods of its waxing and waning form the basis of a large number of of the oldest calendars. Tally sticks, notched bones dating as far back as 20–30,000 years ago, are believed by a few to mark the phases of the Moon. The ~30-day month is an approximation of the lunar cycle. The English noun month and its cognates in additional Germanic languages stem from Proto-Germanic *mǣnṓth-, which is connected to the above-mentioned Proto-Germanic *mǣnōn, indicating the usage of a lunar calendar among the Germanic peoples (Germanic calendar) prior to the adoption of a solar calendar. The PIE root of moon, *méh1nōt, derives from the PIE verbal root *meh1-, "to measure", "indicat[ing] a functional conception of the moon, i.e. marker of the month" (cf. the English words measure and menstrual), and echoing the Moon's importance to a large number of ancient cultures in measuring time (see Latin mensis and Ancient Greek μείς (meis) or μήν (mēn), meaning "month"). Most historical calendars are lunisolar. The 7th-century Islamic calendar is an exceptional example of a purely lunar calendar. Months are traditionally determined by the visual sighting of the hilal, or earliest crescent moon, over the horizon.

Modern art and literature

The Moon has been the subject of a large number of works of art and literature and the inspiration for countless others. It is a motif in the visual arts, the performing arts, poetry, prose and music.

Lunacy

The Moon has long been associated with insanity and irrationality; the words lunacy and lunatic (popular shortening loony) are derived from the Latin name for the Moon, Luna. Philosophers Aristotle and Pliny the Elder argued that the full moon induced insanity in susceptible individuals, believing that the brain, which is mostly water, must be affected by the Moon and its power over the tides, but the Moon's gravity is too slight to affect any single person. Even today, people who believe in a lunar effect claim that admissions to psychiatric hospitals, traffic accidents, homicides or suicides increase throughout a full moon, but dozens of studies invalidate these claims.

 

 

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